The present invention relates to processes for the manufacture of metallic tubing. It is especially concerned with the processing of zirconium alloy nuclear fuel cladding and the cladding produced thereby.
Zircaloy-2 and Zircaloy-4 are commercial alloys, whosw main usage is in water reactors such as boiling water (BWR), pressurized water (PWR) and heavy water (HWR) nuclear reactors. These alloys were selected based on their nuclear properties, mechanical properties and high temperatures aqueous corrosion resistance.
The history of the development of Zircaloy-2 and 4 is summarized in: Stanley, Kass "The Development of the Zircaloys " published in ASTM Special Technical Publication No. 368 (1964) pp. 3-27, and Rickover et al. "History of of the Development of Zirconium Alloys for use in Nuclear Reactors", NR:D:1975. Also of interest with respect to Zircaloy development are U.S. Pat. Nos. 2,772,964; 3,097,094 and 3,148,055.
The commercial reactor grade Zircaloy-2 alloy is an alloy of zirconium comprising about 1.2 to 1.7 weight percent tin, about 0.07 to 0.20 weight percent iron, about 0.05 to 0.15 weight percent chronium and about 0.03 to 0.08 weight percent nickel. The commercial reactor grade Zircaloy-4 alloy is an alloy of zirconium comprising 1.2 to 1.7 weight percent tin, about 0.18 to 0.24 weight percent iron, and about 0.07 to 0.13 weight percent chromium. Most reactor grade chemistry specifications for Zircaloy-2 and 4 conform essentially with the requirements published in ASTM B350-80 (for alloy UNS No. R60802 and R60804, respectively). In addition to these requirements the oxygen content for these alloys is typically required to be between 900 and 1600 ppm, but more typically is about 1200 .+-.200 ppm for fuel cladding applications. Variations of these alloys are also sometimes used. These variations include a low oxygen content alloy where high ductility is needed (e.g. thin strip for grid applications). Zircaloy-2 and 4 alloys having small but finite additions of silicon and or carbon are also commercially utilized.
It has been a common practice to manufacture Zircaloy (i.e. Zircaloy-2 and 4) cladding tubes by a fabrication process involving: hot working an ingot to an intermediate size billet or log; beta solution treating the billet; machining a hollow billet; high temperature alpha extruding the hollow billet to a hollow cylindrical extrusion; and the reducing the extrusion to substantially final size cladding through a number of cold pilger reduction passes (typically 2 to 5 with about 50 to about 85% reduction per pass), an alpha recrystallization anneal prior to each pass. The cold worked, substantially final size cladding is then final alpha annealed. This final anneal may be a stress relief anneal, partial recrystallization anneal or full recrystallization anneal. The type of final anneal provided is selected based on the designer's specification for the mechanical properties of the fuel cladding. Examples of these processes are described in detail in WAPD-TM-869 dated 11/79 and WAPD-TM-1289 dated 1/81. Some of the characteristics of conventionally fabricated Zircaloy fuel cladding tubes are described in Rose et al. "Quality Costs of Zircaloy Cladding Tubes" (Nuclear Fuel Performance published by the British Nuclear Energy Society (1973), pp. 78.1-78.4).
In the foregoing conventional methods of tubing fabrication the alpha recrystallization anneals performed between cold pilger passes and the final alpha anneal have been typically performed in large vacuum furnaces in which a large lot of intermediate or final size tubing could be annealed together. Typically the temperatures employed for these batch vacuum anneals of cold pilgered Zircaloy tubing have been as follows about 450 to about 500.degree. for stress relief annealing without significant recrystallization; about 500.degree. C. to about 530.degree. C. for partial recrystallization; annealing; and about 530.degree. C. to about 760.degree. C. (however, on occasion alpha, full recystallization anneals as high as about 790.degree. C. have been performed) for full alpha reorystallization annealing. These temperatures may vary somewhat the degree of cold work and the exact composition of the Zircaloy being treated. During the foregoing batch vacuum alpha anneals it is typically desired that the entire furnace load be at the selected temperatures for about one to about 4 hours, or more, after which the annealed tubes are vacuum or argon cooled.
Additional examples of the conventional Zircaloy tubing fabrication techniques, as well as variations thereon, are described in the following documents: "Properties of Zircaloy-4 Tubing" WAPD-TM-585; Edstrom et al. U.S. Pat. No. 3,487,675; Mantinlassi U.S. Pat. No. 4,233,834; Naylor U.S. Pat. No. 4,090,386; Hofvenstam et al U.S. Pat. No. 3,865,635; Anoersson et al. "Beta Quenching of Zircaloy Cladding Tubes in Intermediate or Final Size," Zirconium in the Nuclear Industry: Fifth Conference. ASTM STP754 (1982) pp. 75-95.; McDonald et al. U.S. patent application Ser. No. 571,122 a continuation of Ser. No. 343,787, filed Jan. 29, 1982 now abandoned and assigned to the Westinghouse Electric Corporation); Sabol et al. U.S. patent application Ser. No. 571,123 (a continuation of Ser. No. 343,788, filed Jan. 29, 1982, now abandoned and assigned to the Westinghouse Electric Corporation; Armijo et al. U.S. Pat. No. 4,372,817; Rosenbaum et al. U.S. Pat. No. 4,390,497; Vesterlund et al U.S. Pat. No. 4,450,016; Vesterlund U.S. Pat. No. 4,450,020; and Vesterlund French patent application Publication No. 2,509,510 published Jan. 14, 1983.
Included among the foregoing processes and resulting fuel cladding designs are those which include the use of an annular liner of a first material for improved PCI resistance (Pellet Cladding Interaction) bonded to an outer relatively stronger annular layer of a second material which provides the cladding with its aqueous corrosion resistance and required mechanical properties.
There exists a need in the art for a nuclear fuel cladding, and a process for producing that cladding, in which the cladding is made of a single alloy having a combination of the required PCI resistance aqueous corrosion resistance and mechanical properties.